Problem:
.gp files generated by mummerplot contain a deprecated syntax that does not allw one to replot using the files.
that is
gnuplot < xxx.gp
will fail.

The offending line is
set ticscale 0 0
which is now no longer valid gnuplot syntax

the correct syntax should be
set tic scale 0,0

Regrerssion:
for some reason I don't understand mummerplot is able to actually generate the desired plot, even though the .gp file it generates does not itself work. When Mummerplot does generate the plot in X or Postscript there is a warning message about this syntax. But the plot actually is created. Not so when one tries to load the .gp file directly into gnuplot.
I don't know what the difference is.

workaround: edit the .gp file

Also a suggestion:
when the .gp file is generated it would be a lot more useful if it were generated agnostically about where the output is to be directed. That is to say if someone uses mummerplot to generate a postscript file then they have the postscript file already and don't need the .gp file unless of course they wanted to use it to generate something other than a postscript file. Thus having the .gp file set the output ot postscript is exactly what you don't want in that case. Conversely, if one displayed the plot in X then one might wnat to later generate a postscript file. Having the .gp file direct the output to the X window again defeats this goal. Simply having the .gp file be agnostic about it's output woul dbe more useful.